Semiconductor memory devices may be classified into and read only memory (ROM) products and RAM products, such as a dynamic random access memory (DRAM) and static random access memory (SRAM). RAM products may have relatively fast data input/output performance while being volatile to lose data according to the passage of time. ROM products may have relatively slow data input/output performance while being nonvolatile to retain data if the data is input. ROM products can be classified into a ROM, a programmable ROM (PROM), an erasable PROM (EPROM), and an electrically EPROM (EEPROM). Among the ROM products, the demand for the EEPROM that is electrically programmable or erasable has been increased. The EEPROM or a flash EEPROM equipped with a simultaneous erase function may have a stack structure of a floating gate and a control gate. Such a flash memory cell has been extensively used for a printer, a computer BIOS, and portable electronics such as a notebook personal computers, PDAs and cellular phones. In such nonvolatile semiconductor memory devices, various structures have been suggested in order to overcome problems caused by a short channel effect as a channel length is shortened.